Just want you to know that I am going in 2 weeks to have a lump in my groin removed. same as the one you described, golf-ball sized, mine 's been there 9 months, mine's actually fixed to muscle or something in there. My doc told me he's highly suspicious of lymphoma and he considers me a "very healthy young lady", despite all my health complaints over the last year. PLEASE keep us posted about the results of the biopsy. since I can't find information on any othe disease but cancer involving a massively large lymph node. your optimism is very uplifting

best of luck.
ps- my doctor told me there is a very good chance of losing some sensation to touch in that area after the surgery which might come back, might not.